Some experimental applications require that a repetitive voltage wave form be extracted from random electrical noise, sometimes from DC off-set drifts. Such applications include measurements of pulsed nuclear resonance, laser excitation delay, ultrasonic echoes and other evoked-response phenomena. It is known that a boxcar integrator may be able to recover such a signal buried in random electrical noise by repeatedly sampling the signal plus noise at an appropriate time delay sampling the signal plus noise at an appropriate time delay after the trigger pulse which initiates the evoked response. When many such samples are integrated, the random noise will tend to integrate to zero. The output signal-to-noise ratio will be improved compared to the input signal to noise ratio by a factor approximately equal to the square-root of the number of samples taken. If a slow DC drift causes errors, a dual-channel boxcar integrator, also called a time domain difference amplifier, is usually used with one channel providing the signal plus DC information and the other channel providing just the DC information. By subtracting the one signal from the other, the signal information is obtained.
In ultrasonic echo detection the information to be recovered includes the time delay between the trigger and the signal pulse or pulses, which is a function of some parameter of interest as well as the signal pulse amplitude. A manual search for the signal each time that the time delay changes may then become a very time-consuming effort, susceptible to error. To perform such a search, the operator must set the time delay of the boxcar integrator and wait while a number of integrations are taken. If the signal is not found, the time delay must be manually changed and a new set of samples must be integrated. This must be repeated until the signal is found. Even if operator errors are neglected, the time and operator attention required minimize the possibility of making such measurements efficiently. Prior art boxcar integrators with a sweep capability are available, but they do not provide the precise time interval or signal amplitude measurements needed.
It is therefore an object of this invention to provide an automatically sweeping circuit for searching for a signal response in an output signal which is evoked in response to a trigger input.
Another object of this invention is to provide an automatically sweeping boxcar integrator to automatically search for echoes hidden in noise.